This invention relates generally to methods and apparatus for CT imaging and other radiation imaging systems and, more particularly, to z-smoothing for helical image artifact reduction.
In at least some xe2x80x9ccomputed tomographyxe2x80x9d (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at a detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged, so the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent to the scintillator. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield units,xe2x80x9d which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a xe2x80x9chelicalxe2x80x9d scan may be performed. To perform a xe2x80x9chelicalxe2x80x9d scan, the patient is moved in a z-axis direction synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reducing scan time, helical scanning provides other advantages such as better use of injected contrast, improved image reconstruction at arbitrary locations, and better three dimensional images.
The x-ray beam is projected from the x-ray source through a pre-patient collimator that defines the x-ray beam profile in the patient axis, or z-axis. The collimator typically includes x-ray-absorbing material with an aperture therein for restricting the x-ray beam. In at least one known CT imaging system, a scanning mode and corresponding reconstruction method are implemented for 3:1 and 6:1 helical pitches. The 6:1 helical pitch mode is referred to as a xe2x80x9chigh speedxe2x80x9d (HS) mode because volume coverage is large, and scanning is faster along z-axis than in the 3:1 helical pitch mode.
In some CT imaging systems, the detector array is segmented so that a plurality of quasi-parallel slices of projection data are acquired and processed to construct a plurality of images corresponding to several slices though a volume. Such CT imaging systems are referred to as xe2x80x9cmultislicexe2x80x9d systems. Multislice systems provide more data available for reconstructing an image.
In one exemplary embodiment of a multislice scanner, N rows, e.g., N slices, of projection data are acquired for each view angle. A range of pitches exists for which measurements are available at least at two source locations. Measurements acquired at different source positions are known as xe2x80x9cconjugate measurements.xe2x80x9d Pitches for which conjugate measurements are available are known as xe2x80x9cHigh Qualityxe2x80x9d (HQ) pitches. When an HQ pitch is used, a CT imaging system is said to operate in xe2x80x9cHQ mode.xe2x80x9d For a range of pitches for which majorities of measurements are available at only one source position are known as xe2x80x9cHigh Speedxe2x80x9d (HS) pitches. When an HS pitch is used, a CT imaging system is said to operate in xe2x80x9cHS mode.xe2x80x9d The availability of multislice data allows interpolation/extrapolation from different rows at the same source position. Higher HS pitches are made possible by allowing data extrapolation. Increasing the number of slices in a multislice imaging system, however, leads to increased cone angles which have a detrimental effect on image quality.
At least one known CT imaging system utilizes a detector array and a xe2x80x9cdata acquisition systemxe2x80x9d (DAS) for collecting image data. The detector array includes detector cells or channels that each produce an analog intensity signal representing x-ray energy impinging on the detector cell. The analog signals are then converted by the DAS to digital signals, which are used to produce image data. Detector cell degradation, as measured by gain non-linearity, typically produces ring or band annoyance artifacts.
Methods and apparatus are provided for reducing image artifacts from a computed tomography (CT) image generated using a CT system. In one aspect a method for reconstructing a computed tomography image of an object is provided which comprises scanning an object utilizing a step-and-shoot mode to acquire a plurality of projection views of the object, reconstructing an image based on projection data from a particular detector row, determining a smoothing function based on a cone angle of the detector row and an image pixel distance from an iso-center, and applying the smoothing function to the reconstructed image.
In another aspect, a method is provided for reconstructing a computed tomography image of an object which comprises scanning an object utilizing helical mode to acquire a plurality of projection views of the object, reconstructing an image based on projection data from a plurality of detector rows, determining a z-smoothing function based on at least one of a helical pitch, an image pixel distance from an iso-center, a number of detector rows, a reconstruction algorithm, and a slice thickness for the particular detector row, and applying the z-smoothing function to the reconstructed image and applying the z-smoothing function to pixels within the reconstructed image, the amount of smoothing applied to an individual of image pixel in the reconstructed image based on a location of the pixel from the iso-center.
In still another aspect, an imaging system is provided which comprises a computer, a gantry having a detector array, an x-ray source for radiating an x-ray beam along an imaging plane toward a detector array including a plurality of detector cells, the computer coupled to the x-ray source and the gantry, where the imaging system is configured to scan an object utilizing step-and-shoot mode to acquire a plurality of projection views of the object, reconstruct an image based on projection data from a particular detector row, determine a smoothing function based on a cone angle of each detector row and an image pixel distance from an iso-center, and apply the smoothing function to the reconstructed image.
In yet another aspect, an imaging system is provided which comprises a computer, a gantry having a detector array, an x-ray source for radiating an x-ray beam along an imaging plane toward a detector array including a plurality of detector cells arranged in rows, the computer coupled to the x-ray source and the gantry, where the imaging system is configured to scan an object utilizing helical mode to acquire a plurality of projection views of the object, reconstruct an image based on acquired projection data, determine a z-smoothing function based on at least one of a helical pitch, an image pixel distance from an iso-center, a number of detector rows, a reconstruction algorithm, and a slice thickness for each particular detector row, and apply the z-smoothing function to pixels within the reconstructed image, the amount of smoothing applied to an individual of image pixel in the reconstructed image based on a location of the pixel from the iso-center.
In another aspect a processor is provided which is programmed to reduce image artifacts in a computed tomography system. The processor is configured to scan an object to acquire a plurality of projection views of an object, define a measurement plane, define a plane of reconstruction perpendicular to a patient axis that intersects the measurement plane at an iso-center, reconstruct an image from the plurality of projection views, determine an image pixel distance to the iso-center and a cone angle of detector rows, and apply a z-smoothing weights, based upon the image pixel distance from the iso-center, to at least one detector channel in a z-axis direction to reduce image artifact.
In yet another aspect, a computer-readable medium in an imaging system is provided which comprises a function to acquire projection data for a plurality of detector rows, a function to reconstruct images from the projection data, a function to determine a cone angle for each detector row and an image pixel distance to an iso-center, and a function for applying a z-smoothing function, based upon the image pixel distance from the iso-center, to the reconstructed images to reduce image artifact.